Host and Parasite Interactions During Single and Concurrent Infections withEimeria nieschulziandE. separatain the Rat1

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Abstract
SYNOPSIS.Some effects of 2 species ofEimeriain the rat were studied in single and concurrent infections. Groups included singly infectedE. nieschulziandE. separatacontrols (15 rats each) and 4 groups of 5 rats each infected withE. separata(shorter life cycle) at different days of an establishedE. nieschulziinfection. Experiments were conducted using inoculations of approximately 3,000, 5,000, and 7,000 sporulated oocysts of each species. Hosts showed consistently poor weight gain on day 3 postinoculation (PI) inE. separatainfections and usually showed weight loss on days 7 and/or 8 PI inE. nieschulziinfections. Spleen weights indicated that the hosts responded immunologically to both parasites. Small intestine weights increased duringE. nieschulziinfections as did cecum‐colon weights duringE. separatainfections. In severe infections both species seemed to cause pathologic change in tissues at a distance from the site of endogenous development.Fecal collections were made at 24‐hr intervals from the time of 1st inoculation till the end of patency. Patency usually began on day 7 PI forE. nieschulziand on day 4 PI forE. separatawith oocyst discharge peaking on days 8 and 5, respectively. In 2 instances, a biphasic pattern of oocyst discharge was found forE. separataduring concurrent infections. The occurrence of interspecific interactions was indicated by an increased discharge of oocysts ofE. separata, including those with the ability to sporulate, in doubly‐infected animals as compared with those in singly‐infected controls. It was suggested that the increased oocyst discharges may have resulted from additional production of merozoites during schizogony. Concurrent infections weakened the host more than single infections and this seemingly allowed more multiplication of the parasites.